1. Field of the Invention
The present invention relates to an electric double-layer capacitor and more particularly, to an electric double-layer capacitor having positive and negative polarizable electrodes made of a polarizable solid material such as activated-carbon.
2. Description of the Prior Art
An electric double-layer capacitor is a capacitor utilizing an electric double-layer generated at an interface of a solid (i.e., a polarizable electrode) and an electrolyte solution. This capacitor has a feature that a large capacitance in the order of farad (F) is readily realized, which is due to the fact that the electric double-layer equivalent to a dielectric layer in a popular capacitor is approximately as small as a molecule diameter.
FIGS. 1 and 2 show a conventional electric double-layer capacitor.
As shown in FIG. 2, this conventional electric double-layer capacitor 109 has a stacked structure 106 comprised of rectangular plate-shaped basic cells 105. These basic cells 105 are stacked in a direction perpendicular to the cells 105, and are electrically connected in cascade. Each of the basic cells 105 is a unit of stacking.
A pair of external terminal plates 108 are attached onto the outermost basic cells 105 of the stacked structure 106, respectively. A pair of rubber plates (not shown) serving as spacers are attached onto the pair of external terminal plates 108, respectively. A pair of pressing plates 107 are attached onto the pair of rubber plates, respectively.
Connection bolts (not shown) are inserted to penetrate the stacked structure 106 of the basic cells 105, the external terminal plates 108, the pair of rubber plates, and the pair of pressing plates 107 at the four corners of the structure 106. Nuts (not shown) are engaged with the screws formed at both ends of the connection bolts to press the stacked basic cells 105 along the bolts, thereby holding or combining the stacked basic cells 105, the external terminal plates 108, the pair of rubber plates, and the pair of pressing plates 107 together under pressure.
Due to the applied pressure, the contact resistance across the adjacent basic cells 105 and that across the outermost basic cells 105 and the corresponding terminal plates 108 are kept low.
As shown in FIG. 1, each of the basic cells 105 is comprised of a pair of rectangular plate-shaped, positive and negative polarizable electrodes 104, a non-electron-conductive, porous sheet-like separator 102, an electrically insulating, tubular gasket 103, and a pair of electrically conductive sheet-like collectors 101.
An electrolyte solution 113 such as a water solution of sulfuric acid is absorbed onto the polarizable electrodes 104. The separator 102 is sandwiched between the pair of electrodes 104. The pair of electrodes 104 and the separator 102 are placed in a tubular inner space 116 of the gasket 103. The collectors 101 are placed on both ends of the gasket 103 to close its opening ends, respectively.
The electrolyte solution 113 is injected and stored in the tubular inner space 116 of the gasket 103. The pair of electrodes 104 and the separator 102 are immersed in the electrolyte solution 113. The solution 113 is sealed by the gasket 103 and the pair of collectors 101.
The pair of collectors 101, which are fixed onto the respective electrodes 104, are made of an electrically conducive rubber or plastic containing an electrically conducive carbon powder. The pair of collectors 101 serve not only as terminal plates of the basic cell 105 but also as sealing members for the electrolyte solution 113 together with the gasket 103.
The pair of polarizable electrodes 104 are, for example, made of a solid activated-carbon/polyacen composite material, which is disclosed in the Japanese Non-Examined patent Publication No. 4-288361 published in 1992. The pair of electrodes 104 are opposed to one another through the separator 102.
The separator 102 is made of porous glass fibers used for a lead-acid battery, which is non-electron-conductive and ion-permeable.
The pair of external terminal plates 108 are attached onto the outermost collectors 101 of the basic cells 105 of the stacked structure 106, respectively.
Generally, the above-identified basic cell 105 independently exhibits a charge-storage (i.e., capacitor) function and as a result, the single cell 105 may be used as an electric double-layer capacitor. However, actually, a plurality of the basic cells 105 are often connected in cascade to thereby constitute the stacked structure 106, as shown in FIG. 2. The purpose of this stacked structure 106 is to provide a sufficient dielectric strength against the supply voltage for an electronic circuitry in which the electric double-layer capacitor 109 is used.
In recent years, novel application fields of electric double-layer capacitors have been found and studied because of the increased capacitance and the decreased Equivalent Series Resistance (ESR). For example, an electric double-layer capacitor is used as an auxiliary power supply for driving a starter motor of an automobile together with a lead-acid battery, as an auxiliary power supply for coping with instantaneous interruption of various systems, and as an auxiliary power supply for assisting a solar battery.
In these novel application fields, it is required that the electric double-layer capacitors serve to supply electric power as long as possible. It is indispensable for the electric double-layer capacitors to ensure a sufficiently long operation period even at a large discharge current.
From this point of view, the conventional electric double-layer capacitor 109 shown in FIGS. 1 and 2 does not have a satisfactorily large capacitance. If the thickness of each polarizable electrode 104 is increased, the capacitance may be increased. In this case, however, the ions of the electrolyte solution 113 existing within the micro pores of the electrodes 104, which have formed electric double-layers together with electric charges existing in the polarizable electrodes 104 through a charge process, need to move a longer distance out of the micro pores during a discharge process.
This causes a problem that the ions existing in the micro pores of the electrodes 104 near the collectors 101 are unable to follow the movement or diffusion of the remaining ions, because the ions existing near the collectors 101 are difficult to quickly diffuse from the micro pores toward the separator 102 through the macro pores of the electrodes 104. In other words, the ions existing in the micro pores located near the collectors 101 are not used for a discharge process at a large current.
Thus, the diffusion rate or velocity of the ions existing in the micro pores becomes a rate-determinate factor in a discharge process, thereby decreasing apparently the capacitance of the conventional electric double-layer capacitor 109. This results in degradation of the capacitor function.
To solve this problem of the capacitor function degradation, an improved electric double-layer capacitor was developed, which is disclosed in the Japanese Non-Examined Patent Publication No. 3-201519 published in September 1991.
FIG. 3 shows one of basic cells of the improved electric double-layer capacitor 105' disclosed in the Japanese Non-Examined Patent Publication No. 3-201519.
In the electric double-layer capacitor 105' in FIG. 3, a pair of polarizable electrodes 104' are used instead of the pair of polarizable electrodes 104 in FIG. 1.
Each of the polarizable electrode 104' is formed by a porous sintered material consisting of first and second activated carbons with different densities, in which each of the first and second activated carbons is made of sintered activated-carbon particles. The pair of electrodes 104' are placed in such a way that the second activated carbon having the higher density is contacted with the pair of collectors 101 and the first activated carbon having the lower density is contacted with the separator 102.
The polarizable electrode 104' is produced by a step of stacking a layer of a first powder of activated-carbon particles and a layer of a second powder of activated-carbon particles in a sintering mold, and a step of sintering the layers of the first and second powders under heat and pressure in the mold. The activated-carbon particles of the first and second powders are different in average particle size.
With the improved electric double-layer capacitor using the basic cell 105' shown in FIG. 3, the above-identified problem of the capacitor function degradation is solved without increasing the thickness of the polarizable electrode 104' with respect to the polarizable electrode 104 shown in FIG. 1. In this case, however, the necessary moving distance itself of the ions in the electrolyte solution 113, which have formed the electric double-layers, during a discharge process is not decreased. Therefore, the obtainable improvement is not satisfactory.
Additionally, with the improved electric double-layer capacitor using the basic cell 105' shown in FIG. 3, the polarizable electrode 104' is produced by successively forming the layers of the first and second powders of activated-carbon particles in a sintering mold, and then, by sintering the layers of the first and second powders under heat and pressure. Therefore, the polarizable electrode 104' tends to have a camber or crack, or to be split due to the difference in shrinkage degree of the first and second powders during the sintering process. Thus, the fabrication yield becomes low.
Moreover, since the activated-carbon particles forming the polarizable electrode 104' is at most several hundreds micrometers in diameter, it is difficult to discriminate between the high- and low-density sides by the naked eye. To avoid such the difficulty, some mark may be provided on the high- or low-density side of the electrode 104'. However, this necessitates a confirmation step of the mark in the assembly processes and results in addition of an unnecessary step.
Consequently, not only the fabrication yield of the capacitor but also the productivity thereof will degrade.